The present invention relates to a method and a device to determine the passing of a reflector point to a pre-selected distance by indirect measurement of the propagation delay of a phase-modulated and/or frequency-modulated continuous wave reflected at said point.
In a certain number of fields, a known way to make precise measurements of the distance to a point being aimed at is to measure the travelling time needed for a wave, notably an electromagnetic wave, to return to a transmitter after reflection from the point aimed at. To avoid the use of transmitters with high instantaneous power, systems of continuous transmission in time have been developed, in such a way as to preserve a sufficient level of energy illuminating the point aimed at. The reception therefore takes place simultaneously with the transmission, and to find the time taken for the to-and-fro propagation of the wave, there is provision for the phase and/or frequency coding of the transmitted wave and for determining the delay between the transmitted code and the received code. Systems such as these are found, for example, in the fields of radio-altimetry, altimetrical triggers, anticollision systems for vehicles, etc.
Different codes have been developed for these systems, all based on a modulation of the phase and/or the frequency of the transmitted wave. One of the most commonly used codes is that of the linear modulation of frequency. In this case, the distance is extracted through the multiplying of the received signal by the transmitted signal and through the measuring of the beat frequency obtained. Unfortunately, as it is not a time but a frequency that is directly measured, it is highly possible to conceive of a situation at the point aimed at wherein, between the reception of the wave and its retransmission, there is a frequency shift that falsifies the measurement and hence enables the system to be jammed. It is clear that the same drawback can be found with other known types of coding systems.